528 .MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



those usually met with in corals, measuring fully 3 mm. in length. When first extruded they 

 were densely opaque, and the posterior end (oral) was deeply pigmented. An hour or two after 

 being set free some became greatly distended at the aboral pole, and as a consequence were 

 more nearly transparent: others, again, became swollen at the oral extremity, the opposite end 

 remaining narrow. 



In the inflated larva represented in tig. 125 three pairs of mesenteries were already indicated, 

 all extending downward from the minute, circular, oral aperture. The members of one 

 pair of mesenteries extended nearly the whole length of the larva, and along their line of 

 attachment were much darker and broader than the others. On one side of the pair were two 

 other faint mesenterial attachments, which continued but a short way down the polyp, while on 

 the other side was a third pair only just apparent, and having a still shorter vertical course. All 

 three pairs, however, start from the uppermost extremity of the polyp. 



Some of the larvae immediately on extrusion were preserved in formol, and others in 

 corrosive acetic, when they threw out a quantity of mucus, which resulted in the adherence of 

 minute foreign particles. The distended larvae nearly always collapsed during the process of 

 preservation. 



Transverse sections of the freshly extruded specimens reveal that the larva 1 are practically 

 solid bodies, the interior being tilled with a compact vacuolated tissue, bearing numerous nuclei 

 and zooxanthelhv. Boundaries in the vacuolar endoderm are indicated toward the middle, in 

 association with the mesenteries, and in the middle of some of the larvae there is a faint indication 

 that the endodermal tissue is beginning to break down, but as yet they are practically solid. 



Toward the oral extremity the endoderm is crowded with zooxanthella?, which are only 

 sparingly distributed elsewhere. An examination of the outer ectoderm reveals comparatively 

 few algae, and these are scattered somewhat uniformly throughout the layer. The strong 

 pigmentation of the oral extremity, noticed among the external characters, is manifestly due to 

 the accumulation of zooxanthella' within the oral endoderm, not. as is more usually the case, 

 to their presence in huge numbers in the ectoderm. 



Both longitudinal and transverse sections through the stomodaeum indicate the absence of 

 any actual lumen in the tube, and the compact character of the interior of the larva, above 

 described, is not such as to suggest that the circulation of any internal nutrient fluid had been 

 established up to the moment of liberation. 



The uppermost sections through the oral extremity reveal the presence of three pairs of 

 mesenteries, all extending from the outer wall to the stomodaeum. The ventral pair, however, 

 is represented only by the merest rudiments, and the dorsal pair extends but a short distance; 

 neither pair stretches downward the full length of the stomodaeum. The middle of the three 

 pairs is by far the most important; its members are inserted on the stomodanim throughout its 

 extent, and when they become free the edge is tipped with a mesenterial filament which appears 

 as a deeply-staining tissue, wholly resembling that of the stomodaeal ectoderm: the two are in 

 absolute continuity with one another, and in every way seem one and the same tissue. The 

 mesenterial filament extends nearly two-thirds the length of the polyp, and is very conspicuous in 

 sections on account of the deeply-staining character of its constituent cells. The other 

 mesenterial pairs present no indications of filaments. 



/. dipsacea is of interest as showing the early stage at which the second and third pairs of 

 mesenteries are united with the stomodaeum; indeed, they seem to originate at the angle between 

 the wall and stomodaeal invagination, and thence grow down the column and the stomodaeum. 



The ectoderm is characterized by numerous large clear gland cells, which give out their 

 mucus when the larva' are preserved. Many large nematocysts are also present, and the aboral 

 extremity displays a strongly developed nerve layer. 



LARVA AND YOUNG POLYPS OF FAYIA FRAGUM. 

 ( Pis. NIII-XV, figs. 96-116.) 



The polyps of several colonies of this species collected around Port Henderson, early in 

 April, were charged with larvae, which were extruded singly from time to time. Occasionally, 

 an unfertilized egg would also appear. The larvae could be seen through the transparent walls 



